1. Field of the Invention
The present invention relates to a process for preparing supported superacid catalysts active in alkylation of aromatic hydrocarbons with alkylating agents, more particularly with olefinic agents.
2. Description of the Prior Art
A catalyst currently used for alkylation reactions is aluminum trichloride. In actual practice the aromatic hydrocarbon and the alkylating agent are contacted in a reaction medium containing a fluid catalytic complex formed by aluminum trichloride, hydrocarbons and a hydrogen halide. This procedure involves many drawbacks such as loss of catalyst by solubilization phenomena in the organic reaction medium entailing expensive treatments in order to remove the dissolved catalyst from the organic medium. Moreover, serious corrosion problems arise due to the acidic properties of the catalytic complex. Similar difficulties are met with in the use of further fluid catalytic complexes, such as those of boron trifluoride and similar compounds.
U.S. Pat. No. 3,050,570 describes an alkylation process carried out in the presence of a catalyst consisting of silica-magnesium oxide having adsorbed thereon boron trifluoride. According to this Patent, the liquid alkylation medium contains dissolved boron trifluoride in a proportion of 1 to 2 grams per mole of the olefinic alkylating agent in order to maintain significant reaction rates. The drawbacks described above are thereby not avoided.
Various metal halides of the Lewis acids type or the corresponding Bronsted acids are further known as alkylation catalysts.
Thus, for instance, U.S. Pat. No. 3,708,553 describes alkylation catalysts of the MX.sub.n type, wherein M is a metal belonging to Groups IVB, V or VIB of the Periodic System of Elements, X is a halogen and n an integer from 3 to 6. These catalysts can also be prepared in a supported form, that is adsorbed on inert supports. However, their use for alkylation necessitates the presence of free catalyst in the liquid reaction medium.
No supported catalyst has been found heretofore, in which the Lewis or Bronsted acids remain permanently fixed on the support, and which is so active that it can be commercially exploited in alkylation reactions.
Consequently, known processes do not avoid the drawbacks consisting in loss of catalyst in the organic reaction phase and purification of the organic phase, and corrosion problems have not found a satisfactory solution either.
The above drawbacks are avoided by the supported superacid catalysts obtained by the process of the invention allowing the obtention of alkylates totally free of catalytic residues, which can therefore be directly conveyed to distillation without any preliminary treatment. This circumstance also solves the problems relating to corrosion effects.